Manipulators, for example industrial robots, are intended to move to poses highly precisely, as a rule many times, in order to bring a tool to a desired position.
Manipulator arms and joints can heat up, in particular due to heat emission from the drive motors, friction in the manipulator kinematics, but also environmental influences. The accompanying change in shape, in particular thermal expansion, results in the tool no longer occupying the same position, despite the same joint position, i.e., theoretically the identical pose.
It is therefore known, for example from DE 10 2004 024 378 A1, to measure the manipulator relatively, in order to register the deviation of its kinematics during operation. In that process the manipulator moves in regular recalibration cycles to the same joint positions, i.e., theoretically the same pose, and registers a spherical reference body by using a manipulator-guided sensor. If the registered position of the stationary reference body changes relative to the manipulator-guided sensor, this can be used to ascertain the change in the manipulator kinematics and can be compensated for accordingly.
Besides this relative measurement, absolute measurement of manipulators is also known, in order to calibrate them or to determine the parameters of a manipulator model on which a manipulator control system is based. For example, if a trajectory of an industrial robot is planned offline in advance, it depends on the exactness of the model parameters how precisely that real robot moves to the theoretically predefined positions. DE 198 26 395 A1 proposes to this end that the position of a manipulator-guided reference body be registered by stationary cameras, and that the model parameters be adjusted on the basis of the deviation of this actual position from a theoretical position determined using the model.